455 ring pack has a 5/64" top ring, 1/16" second ring and 3/16" oil ring. Stock dimensions. The increase in stroke from 4" to 4.210" added significant wear to the second ring, so the size reduction was made before the engine went into production.
The Speed Pro L2359F is FAR superior to the KB piston. We use them (KBs) for ash trays. The Icon forgings, however, are every bit as good, if not better than the Speed Pros, and lighter, for about the same money.
In order for a "flat top" piston'd 455 to make under 9.5:1 (accepted limit for 93 octane and iron heads on a Pontiac) requires 98 CCs in the chamber/dish, not including the gasket. Purge the term "get away with" from your vocabulary. Do it right.
Also, be certain to install new FORGED connecting rods. Don't let ANYONE convince you you don't need them. The original rods are a cast steel, and were inadequate when NEW, much less 40 years later... RPM rods make a nice replacement for a reasonable price (about the same cost as new bolts and resizing the old rods).
Be sure to get Jim Hand's "How to Build Max-performance Pontiac V8s" by SA Designs. It's the most current study of the ol' Injun. There's a "rewrite" in process, but none of the original "players" were consulted. The "tractor motor" crowd has hijacked the book. It's okay. Still TONS of good info on modern builds "out there". And we HAVE learned how to build Pontiacs that rev just like any other brand... (we shifted Dirty Bird @ 9,200, a 475 CID Pontiac blown/alcohol, about 2,200 HP) But they STILL make a bunch more low-end than the "other" GM engines that are popular among the racer crowd.